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United States Patent |
5,576,914
|
Rottmayer
,   et al.
|
November 19, 1996
|
Compact read/write head having biased GMR element
Abstract
A compact read/write head is provided having a biased GMR element. Biasing
of the GMR element provides distinguishable response to the rising and
falling edges of a recorded pulse on an adjacent medium. It also improves
the linearity of the response and helps to reduce noise.
Inventors:
|
Rottmayer; Robert E. (Fremont, CA);
Zhu; Jian-Gang (Roseville, MN)
|
Assignee:
|
Read-Rite Corporation (Milpitas, CA)
|
Appl. No.:
|
337878 |
Filed:
|
November 14, 1994 |
Current U.S. Class: |
360/324 |
Intern'l Class: |
G11B 005/127; G11B 005/33 |
Field of Search: |
360/113,55
|
References Cited
U.S. Patent Documents
4967298 | Oct., 1990 | Mowry | 360/113.
|
5373238 | Dec., 1994 | McGuire et al. | 360/113.
|
Primary Examiner: Evans; Jefferson
Assistant Examiner: Giordana; Adriana
Attorney, Agent or Firm: Kallman; Nathan N.
Claims
What is claimed is:
1. A method for detecting flux transitions of an external excitation field
with a magnetoresistive element having at least first and second
magnetoresistive layers, each layer characterized by a central main
subregion having a magnetic domain orientation, and edge subregions of
magnetic domain orientations, comprising the steps of:
magnetically biasing said magnetoresistive element such that, when the
external excitation field has essentially no strength, the magnetic domain
orientations of said central main subregions of said first and second
magnetoresistive layers define a scissor configuration.
2. A flux detecting method according to claim 1 wherein the angle between
the respective magnetic orientations of the main subregions of said first
and second magnetoresistive layers is approximately 90.degree. for the
case where the external excitation field has essentially no strength.
3. A flux detecting method according to claim 1 wherein said step of
magnetically biasing is such that there is substantially no flipping in
the respective domain orientations of the edge subregions when the
external excitation field transitions from maximum intensity in one
direction to maximum intensity in an opposed second direction.
4. A flux detecting method according to claim 1 wherein the
magnetoresistive element is a giant magnetoresistive element.
5. A flux detecting method according to claim 4 further comprising the step
of:
(b) operating the giant magnetoresistive element in a current perpendicular
to the plane mode.
6. A flux detecting method according to claim 1 wherein said step of
magnetically biasing includes using a combination of a pre-oriented
exchange layer and a soft layer to provide a biasing field.
Description
FIELD OF THE INVENTION
This invention relates to thin film magnetic heads and in particular to a
method of manufacture and to a structure for a thin film magnetic head
incorporating an inductive write head and a magnetoresistive (MR) read
head.
CROSS-REFERENCE TO RELATED APPLICATION
Copending U.S. patent application Ser. No. 08/203,225 which issued as U.S.
Pat. No. 5,446,613 on Aug. 29, 1995, entitled MAGNETIC HEAD ASSEMBLY WITH
MR SENSOR was filed Feb. 28, 1994 on behalf of Robert Rottmayer and is
assigned to the assignee of the present application. The subject matter of
the copending application is related to the present application and is
incorporated herein by reference.
DESCRIPTION OF THE PRIOR ART
Presently known thin film magnetic heads include an inductive write head
and an MR head used for reading recorded signals. Write operations are
carried out inductively using a pair of magnetic write poles. These
magnetic write poles form a magnetic path and define a transducing
nonmagnetic gap in the pole tip region. The poles are in contact at a back
closure region. The transducing gap is positioned to fly close to the
surface of an adjacent recording medium (e.g., a magnetic disk). An
electrical coil or winding is formed between the poles and insulated
therefrom for providing current representative of signal information and
to cause flux flow in the magnetic path of the poles. The varying flux
results in recording of the signal information onto the magnetic medium,
which is moving or rotating closely adjacent to the magnetic head
structure.
Read operations are carried out by a magnetoresistive (MR) element which is
spaced from a pair of magnetic shields. A sensing electric current is
passed through the MR element for sensing the resistance of the MR
element. The resistance of the MR element changes in response to changes
in magnetic flux or transitions received from the adjacent medium. The
shields protect the MR element from stray flux.
Conventionally, the MR element was electrically isolated from the pair of
magnetic shields and a separate set of conductors were provided on one
surface of the MR element to pass a reference current through the MR
element in a so-called `CIP`mode (Current In the Plane mode). The CIP mode
created problems such as shorting due to electromigration. The CIP style
MR element was relatively large in size and expensive to mass produce
because of its complex construction.
More recently, a compact MR head has been developed in which the magnetic
write poles serve also as the shields for the MR element and further as a
means for conducting the MR sense current. The structure and method of
forming such a compact MR head is disclosed in the above-cited U.S. patent
application Ser. No. 08/203,225.
In brief the compact MR head of Ser. No. 08/203,225 may be described as
follows. Thin-film deposition and photolithographic techniques are used to
define on a ceramic substrate, a block-shaped layered structure which has
as its layers, in bottom to top order:
(a) a bottom pole/shield layer made of a magnetically and electrically
conductive material;
(b) a bottom contact layer made of a magnetically nonconductive but
electrically conductive material;
(c) a Giant MR element (GMR) arranged to operate in a mode known as CPP
(Current flowing Perpendicularly through the major Plane);
(d) a top contact layer made of a magnetically nonconductive but
electrically conductive material; and
(e) a top pole/shield layer made of a magnetically and electrically
conductive material that is separated from the bottom pole/shield layer
both by a an operational transducing forward gap and also by a countering
back gap.
The GMR element is typically formed in a patterned multilayer structure,
which may use Cu/Co or Fe/Cr material, for example. In keeping with this
invention, the GMR film is oriented so that the bias current that is
applied to the GMR film is perpendicular to the plane of the film, or in a
Cpp mode.
Read current flows perpendicularly through the GMR element by way of the
electrically-conductive pole/shields and contact layers. A read sense
circuit is coupled to the upper and lower pole/shields for detecting
magnetoresistive fluctuations. Write flux flows across the forward gap
when electric current is passed through an electrical coil formed about
the back gap of the compact GMR head structure.
While the compact GMR head structure of Ser. No. 08/203,225 is highly
advantageous, further improvements can be made. In particular, it has been
discovered that read noise can be reduced and the linearity and gain of
the GMR response can be improved as disclosed below.
SUMMARY OF THE INVENTION
In accordance with this invention, a compact read/write head includes a
magnetically biased GMR element and provides reduced noise in the read
signal and improves the linearity and gain of flux sensing. The MR element
is magnetically biased such that the major domains of the alternating MR
layers define a scissor-type configuration when no excitation field is
supplied by an adjacent medium. The major domain region of a first layer
of the MR element is preferably biased so as to be rotated +45.degree.
relative to its unbiased orientation by the bias field. The major domain
region in an adjacent second layer of the MR element is preferably biased
so as to be rotated -45.degree. relative to its unbiased orientation by
the bias field. This produces an angle difference of approximately
90.degree. between the major domain regions of the first and second layers
due to the bias. The cosine of this bias-induced angle difference
(90.degree.) is approximately zero.
When an excitation field is supplied by the adjacent medium, it rotates the
scissor configuration from the crossed (90.degree.) state towards either a
closed (0) state or an anti-parallel (180.degree.) state, depending on the
polarity of the excitation field. The resultant change in cosine (and
resistance of the MR element which is a function of cosine) is therefore
from zero to a positive one (cosine 0.degree.=+1.0) or to a negative one
(cosine 180.degree.=-1.0). And the resultant change in resistance of the
MR element therefore indicates the polarity of change of the excitation
field.
Additional advantages of the biased into scissor configuration are improved
sensitivity and linear response to low level excitation fields and
reduction in noise from edge effects.
A structure in accordance with the invention includes: (a) a GMR element;
(b) a pair of pole elements positioned about the GMR element for acting as
shields and for simultaneously supplying sense current during a read mode
and for further generating write flux during a write mode; (c) a soft
magnetic element positioned adjacent to the GMR element for applying a
magnetic bias of predefined orientation to the GMR element during the read
mode; and (d) an antiferromagnetic exchange element positioned adjacent to
the soft element for orienting the magnetic domains of the soft element
along said predefined orientation.
A manufacturing method in accordance with the invention comprises the steps
of: (a) providing a substrate; (b) disposing a magnetoresistive element on
the substrate; and (c) further disposing a biasing element on the
substrate, adjacent to the magnetoresistive element, for applying a
magnetic bias to the magnetoresistive element. The bias element can be a
permanent magnet, or a combination of a pre-oriented exchange layer and a
soft layer, or an electric current source which induces the magnetic bias
field, or a combination of one or more of these bias-providing means.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in greater detail with reference to the
drawings in which:
FIG. 1A is a cross-sectional side view showing a biased GMR head structure,
in accordance with the invention;
FIG. 1B is an exemplary front plan view for the biased GMR head structure
of FIG. 1A;
FIG. 2A diagrams a group of snapshots showing micromagnetic domain
distributions in a biased GMR element as the element sweeps past flux
transitions in an adjacent medium;
FIG. 2B is a plot of the cosine-like response of the magnetically biased
GMR element to varying flux intensity;
FIG. 2C is a plot of the polarity-sensitive response of the magnetically
biased GMR element as that element sweeps past a set of flux transitions
in a medium;
FIG. 3A is a cross-sectional side view showing a first step in a method for
manufacturing a biased GMR head in accordance with the invention wherein a
patterned photoresist mask is provided over a plurality of deposited
layers;
FIG. 3B is a cross-sectional side view showing a second step wherein a
cavity is milled;
FIG. 3C is a cross-sectional side view showing a third step wherein an
insulating layer is deposited;
FIG. 3D is a cross-sectional side view showing a fourth step wherein an
antiferromagnetic exchange layer is deposited;
FIG. 3E is a cross-sectional side view showing a fifth step wherein a soft
magnetic layer is deposited;
FIG. 3F is a cross-sectional side view showing a sixth step wherein another
insulating layer is deposited;
FIG. 3G is a cross-sectional side view showing a seventh step wherein the
first photoresist layer is removed;
FIG. 3H is a cross-sectional side view showing an eighth step wherein an
upper pole layer has been deposited and a second photoresist layer has
been deposited and patterned;
FIG. 3I is a cross-sectional side view showing a ninth step wherein
unmasked material is removed;
FIG. 3J is a cross-sectional side view showing a tenth step wherein an
additional insulating layer is deposited and planarized, followed by a
third patterned photoresist layer;
FIG. 3K is a cross-sectional side view showing an eleventh step wherein the
stem portion of the I-beam front view has been patterned, the third
photoresist layer removed, fill material (not shown) has been added about
the stem portion of the I-beam front view, and a wide upper pole layer
thereafter deposited to form the top of the I-beam front view;
FIG. 4 is a cross-sectional side view showing magnetic intercoupling from
the exchange layer to the soft bias layer and from there into the GMR
element;
FIG. 5A is a schematic group of snapshots showing micromagnetic domain
distributions in an nonbiased GMR element as it sweeps past a set of flux
transitions in a medium;
FIG. 5B is a plot of the cosine-like response of the nonbiased GMR element
to varying flux intensity;
FIG. 5C is a plot of response by the nonbiased GMR element as it sweeps
past a set of flux transitions in a medium;
FIG. 6 is a comparative plot of the cosine-like responses of the unbiased
and magnetically biased GMR elements to varying flux intensity.
DETAILED DESCRIPTION
FIG. 1A shows a cross-sectional view of a head structure 100 in accordance
with the invention and a magnetic medium 50 adjacent to the head 100.
For purposes of reference, a Cartesian coordinate grid is shown at 30
having an upwardly extending Z axis and an X axis extending to the right.
The Y axis is understood to extend orthogonally relative to the Z and X
axes, inwardly into the plane of FIG. 1A.
Magnetic medium 50 has a plurality of pre-oriented flux regions 51 defined
on its surface, each directed either in the +Z direction or the -Z
direction. For purpose of example, a first transition 52 that is defined
by opposingly-oriented flux regions 51 is shown producing a first fringe
field extending in the +X direction beyond the medium. A second transition
53 that is defined by other opposingly-oriented flux regions 51 is shown
producing a second fringe field extending in the -X direction.
The magnetic medium 50 moves relative to the head structure 100 along the Z
direction (+Z or -Z). The head structure 100 is spaced away from the
magnetic medium 50 in the X direction by an aerodynamically-defined flying
height, H.
As the head structure 100 is passed by transition regions 52 and 53, a
magnetoresistive portion 123 of the head structure 100 detects the flux or
fringe fields and responds by changing its resistance.
A slider-shaped substrate 110 made of a magnetically nonconductive material
such as ceramic forms a bulk portion of the head structure 100 and
provides the aerodynamic lift. For reasons of illustrative simplicity, the
bulk of the aerodynamic substrate 110 is not shown.
The substrate 110 has a substantially planar top surface 111 extending in
the X direction and a medium-facing sidewall 115 cut substantially at
right angles to the top surface 111 so as to extend in the Z direction.
The end of the substrate top surface 111 that meets with the substrate
sidewall 115 is referred to herein as the forward edge 113.
A first pole/shield layer 121, made of a material that is both magnetically
and electrically conductive (an EC/MC material), is formed conformably on
the substrate top surface 111 extending to the forward edge 113. The
material of the first pole/shield layer 121 can be a nickel-iron
composition, such as Permalloy, or a ferromagnetic material with high
permeability. The Z direction thickness of the first pole/shield layer 121
is preferably in the range of 0.5 to 10 microns and more preferably in the
range of 2 to 3 microns.
The abbreviation form, Ex/Mx will be used below to describe the electrical
and magnetic conductivity properties of various materials, with x=C
meaning it is conductive, x=N meaning it is nonconductive, and x=X meaning
it can be either. Thus EN/MC means, electrically nonconductive and
magnetically conductive. EX/MN means the material is either electrically
conductive or nonconductive, but it is magnetically nonconductive.
A first contact element 122, made of an electrically conductive but
magnetically nonconductive material (an EC/MN material), is formed over a
forward portion of the first pole/shield layer 121, near the substrate's
forward edge 113. The first contact element 122 can be composed of one or
a combination of EC/MN materials selected for example from the group
consisting of: copper (Cu), gold (Au), silver (Ag), and alloys of these
metals. The Z direction thickness of the first contact element 122 is
preferably in the range of 100 .ANG. to 2000 .ANG. and more preferably in
the range of 300 .ANG. to 1500 .ANG..
A giant magnetoresistive (GMR) element 123 is formed over the first contact
element 122. The GMR element 123 may be formed, by way of example, by
depositing a plurality of alternating ultra-thin layers of magnetically
conductive and nonconductive materials such cobalt (Co) and copper (Cu),
each layer being approximately 20 Angstroms thick. As known, the electric
resistance of such a GMR element 123 fluctuates when exposed to a
time-varying magnetic flux. Unlike inductive transducers, a
magnetoresistive element is sensitive to the magnitude of a flux
transition rather than to the rate of change of the flux transition. This
gives magnetoresistive elements certain advantages over inductive
transducers, such as insensitivity to disk speed changes.
The overall Z direction thickness of the GMR element 123 is preferably in
the range of 60 .ANG. to 1000 .ANG. and more preferably in the range of
100 .ANG. to 500 .ANG..
A second contact element 124, made of an EC/MN material that is the same or
equivalent to that of the first contact element 122, is formed over the
GMR element 123. The Z direction thickness of the second contact element
124 is substantially the same as that of the first contact element 122.
A second pole/shield layer 125, made of an EC/MC material that is the same
or equivalent to that of the first pole/shield layer 121, is formed over
and the second contact element 124. The Z direction thickness of the
second pole/shield layer 125 is substantially the same as or less than
that of the first pole/shield layer 121.
A third pole/shield layer 126, made of an EC/MC material that is the same
or equivalent to that of the first and second pole/shield layers, 121 and
125, is formed over the second pole/shield layer 125 and extended
backwards (in the +X direction) to define a back gap 130 with the first
pole/shield layer 121. The Z direction thickness of the third pole/shield
layer 126 is substantially the same as or greater than that of the first
pole/shield layer 121.
The back gap 130 is filled with a material that is at least electrically
nonconductive (EN/MX material) and more preferably with a material that is
both magnetically and electrically nonconductive (EN/MN) such as Al.sub.2
O.sub.3, hard-baked resist or BCB (benzocyclobutene available from Dow
Chemical Corp.).
The space at the forward edge 113, between the top of the first pole/shield
layer 121 and the bottom of the second pole/shield layer 125 defines a
forward write gap (G in FIG. 1B). The dimension of the forward gap G is
defined by the combined Z direction thicknesses of the first contact
element 122, the GMR element 123 and the second contact element 124.
The Z dimension of the back gap 130 should be no more than that of the
forward gap and is more preferably made as small as possible while still
assuring electrical insulation between the first and third pole/shield
layers, 121 and 126.
Referring to FIG. 1B, which shows a preferred front plan view of a relevant
portion of the head structure 100 as seen in the Y-Z plane 31, elements
121 through 126 preferably define an I-beam profile. The Y direction width
in the stem portion of the I-beam profile, which is defined by an upper
part of element 121 and elements 122 through 125, is preferably 20 to 100
microns and more preferably in the range of 3 to 5 microns. The bottom and
top caps of the I-beam profile, which are defined respectively by an upper
part of element 121 and element 126, are wider, preferably by a factor of
1.5 to 2 times as much.
The Z direction height of element 125 (the second pole/shield layer) in
FIG. 1B is preferably 1 to 10 times G, and more preferably approximately 3
times G. The Z direction height of the upper, stem-forming portion of
element 121 (the first pole/shield layer) similarly, is preferably 1 to 10
times G, and more preferably approximately 3 times G.
Although not fully shown, a EN/MN fill and planarizing structure 170
composed of one or more materials such as Al.sub.2 O.sub.3, hard-baked
resist or BCB surrounds the I-beam profile from the substrate top surface
111 to at least the bottom of the third pole/shield layer 126. The third
pole/shield layer 126 can also be shrouded by a passivating EN/MN material
if desired.
Referring again to FIG. 1A, the X direction length of elements 122 through
125 is preferably 50 to 200 microns and more preferably in the range of
100 to 150 microns. As seen in FIG. 1A, the first and third pole/shield
layers, 121 and 126, extend in the +X direction beyond sandwiched elements
122 through 125.
A planar coil 140 having electrically conductive winding members such as
indicated at 141-144 is formed about the back gap 130 and electrically
insulated from the first and third pole/shield layers, 121 and 126, by the
EN/MN fill and planarizing structure 170 or another appropriate EN/MN
support structure.
A write circuit 150 connects to opposed ends of the coil 140 (e.g., to
members 142 and 144 in the case where the coil has spiral-shaped top
view), and during a write mode sends electrical current I.sub.W passing in
a first direction (+Y) through winding members 141-142 positioned on a
forward side of the back gap 130 and sends electrical current passing in a
second direction (-Y) through winding members 143-144 positioned on a rear
side of the back gap 130, to thereby induce flux flow through the forward
and back gaps. Changes in flux flow across the forward gap produce the
different magnetic orientations of magnetized regions 51 in the magnetic
medium 50 during a write operation.
A read circuit 160 connects to opposed back ends of the first and third
pole/shield layers, 121 and 126, and during a read mode sends a sensing
electric current I.sub.R passing in the Z direction through sandwiched
elements 122 through 125. Note that the read-sense current I.sub.R flows
perpendicularly through the GMR element 123 thus avoiding the
along-the-plane electromigration problems and magnetic-biasing due-to
parallel-current problems associated with earlier designs based on CIP
operation (Current In the Plane mode).
An electrically nonconductive, magnetic biasing element 180 is positioned
behind the combination of the first contact element 122, the GMR element
123 and the second contact element 124. The biasing element 180 is also
sandwiched between the first and second pole/shield layers, 121 and 125.
Biasing element 180 produces a magnetic biasing field that extends
substantially along the X direction (+X or -X) into the GMR element 123 as
indicated by the left-pointing arrow drawn at 180.
The purpose of the biasing element 180 will be explained by referring to
FIGS. 2A-C and 5A-C. FIGS. 5A-C depict a `before`situation wherein biasing
element 180 is not present while FIGS. 2A-C illustrate an `after`
situation wherein biasing element 180 is present.
Referring to the `before` illustration of FIG. 5A, a computer simulation
was run to determine magnetic domain distribution in a nonbiased GMR
element at the micro-domain level. For simplicity, a 3-layer GMR element
was assumed with magnetic/nonmagnetic/magnetic layer thicknesses of 20
Angstroms each, X and Y length and width dimensions of 0.50 micron each, a
shield-to-shield gap (G) of 0.35 micron, and a flying height (H) of 500
Angstroms. The actual simulation output was color coded. FIG. 5A provides
a rough, black-and-white sketch of the color coded results.
The upper left portion of FIG. 5A shows a first top-view snapshot (S-551)
of magnetization conditions in two adjacent GMR layers, 510 and 520, while
the head is in a first relative position Z.sub.R =1 over a medium
magnetization transition point 53 that produces a fringe field (excitation
field from media) 551 with downward orientation.
It is understood that GMR layers 510 and 520 actually overlap each other
when viewed in the XY plane 32 (they are stacked one on the next in the Z
direction), but in order to see what is happening in each of these GMR
layers, we have separated them apart one above the other in the
illustration. The external excitation field 551 produced by the medium 50
extends in the XY plane from the medium into the overlapped GMR layers 510
and 520.
For simplicity sake, each snapshot will be referred to by the same
reference number as that of its media excitation field, preceded by the
prefix, `S-`. Thus, snapshot S-551 refers to the snapshot in the upper
left of FIG. 5A having media excitation field 551.
The downward-pointing arrow in subregion 511 of snapshot S-551 indicates
the magnetic orientations of domains in a central main portion of the
first GMR layer 510. As seen, the magnetic orientations of subregion 511
are aligned in snapshot S-551 with the magnetic orientation of the
corresponding medium excitation field 551.
The downward-pointing arrow in subregion 521 of snapshot S-551 indicates
the magnetic orientations of domains in a central main portion of the
second GMR layer 520. As seen, the magnetic orientations of subregion 521
are also aligned with the magnetic orientation of the corresponding media
excitation field 551. The angle difference between the central magnetic
orientations, 511 and 521, of the first and second GMR layers is therefore
zero. The cosine of this angle difference is +1.0 (cos{0}=+1.0).
The magnetic orientations of domains in edge subregions of GMR layers 510
and 520 are similarly indicated by arrows. The first GMR layer 510 has
respective left, top and right edge subregions 512, 513 and 514 in
snapshot S-551. The second GMR layer 520 has respective left, top and
right edge subregions 522, 523 and 524 in snapshot S-551. The junctions
between the central (main) subregions 511, 521 of respective GMR layers
510, 520 and their corresponding edge subregions, 512-514 and 522-524, are
defined here as `subregion walls`.
Note that the magnetic orientations of domains in the left/right side edge
subregions 512, 514,522 and 524 are aligned in snapshot S-551 with the
magnetic orientation of the corresponding media excitation field 551
(pointing down). The magnetic orientations of domains in the top edge
subregions 513 and 523 are oriented to opposingly point right and left due
to antiferromagnetic properties of the GMR layers.
At the next snapshot S-552, the head to medium relative position has
advanced to Z.sub.R =2, and the underlying media excitation field 552 has
decreased in intensity but still points down. (Relative positions Z.sub.R
=1, 2, 3, etc. are successive but not necessarily equally spaced from one
another.)
The magnetic orientations of domains in edge subregions 512-514, 522-524
remain unchanged in snapshot S-552. The magnetic orientations of domains
in the central subregion of the first GMR layer 510 have rotated slightly
counterclockwise though, as indicated at 511', due to interaction with top
edge subregion 513. The magnetic orientations of domains in the central
subregion of the second GMR layer 520 have similarly rotated slightly
clockwise as indicated at 521' due to interaction with top edge subregion
523. The angle difference between the central magnetic orientations, 511'
and 521' is now greater than zero but less than 180.degree.. The cosine of
this angle difference is between +1.0 and -1.0.
At the third snapshot S-553, the head to medium relative position has
advanced to Z.sub.R =3, and the underlying media excitation field 553 has
decreased in intensity to zero. The magnetic orientations of domains in
edge subregions 512-514, 522-524 remain unchanged. The magnetic
orientations of domains in the central subregion of the first GMR layer
510 have rotated further counterclockwise as indicated at 511" due to
interaction with top edge subregion 513 and now point to the right. The
magnetic orientations of domains in the central subregion of the second
GMR layer 520 have rotated further clockwise as indicated at 521" due to
interaction with top edge subregion 523 and now point to the left. The
angle difference between the central magnetic orientations, 511" and 521"
is now 180 degrees. The cosine of this angle difference is -1.0
(cos(180.degree.)=-1.0).
At the fourth snapshot S-554, the head to medium relative position has
advanced to Z.sub.R =4, and the underlying media excitation field 554 has
flipped in direction, now pointing upwardly with slight intensity. The
magnetic orientations of domains in edge subregions 512-514, 522-524
remain unchanged due to hysteresis. The magnetic orientations of domains
in the central subregion of the first GMR layer 510 have rotated further
counterclockwise due to interaction with the media excitation field 554
and now point up and to the right. The magnetic orientations of domains in
the central subregion of the second GMR layer 520 have rotated further
clockwise due to interaction with the media excitation field 554 and now
point up and to the left.
At the fifth snapshot S-555, the head to medium relative position has
advanced to Z.sub.R =5, and the underlying media excitation field 555 has
increased in intensity and continues to point upwardly. The magnetic
orientations of domains in edge subregions 514 and 522 remain unchanged
but portions of the edge subregions previously referred to as 512 and 524
begin to flip in response to the strengthened intensity of media
excitation field 555.
More specifically, as seen in snapshot S-555, the subregion walls of
original subregions 512 and 524 have retracted to new configurations 512'
and 524'. New subregions (or `bubbles`) 552 and 554 now appear in the
flipped-over edge subregion portions with magnetic orientations pointing
up instead of down.
Magnetic domains in the edge subregions cannot rotate due to boundary
conditions and instead change their orientations by flipping. The flipping
mechanism advances in a discontinuous manner in accordance with the
well-known Barkhausen effect in each of the GMR layers 510 and 520. At any
given instant, a flip can occur within an edge portion of one of the GMR
layers 510 and 520 without having a symmetrically countering flip
occurring at the same instant of time in the other of GMR layers 510 and
520. This unbalanced flipping of domains at the edge subregions is
believed to introduce a random noise factor into the change of resistance
of the GMR element.
At the sixth snapshot S-556, the head to medium relative position has
advanced to Z.sub.R =6, and the underlying media excitation field 556 has
increased in intensity while continuing to point upwardly. The magnetic
orientations of domains in central subregions 511'" and 521'" have now
rotated into alignment with the media excitation field 556. The magnetic
orientations of domains in edge subregions 514 and 522 now begin to also
change in response to the strengthened intensity of media excitation field
556 and interaction with the domains in central subregions 511'" and
521'".
More specifically, as seen in snapshot S-556, the subregion walls of
original subregions 512 and 524 have retracted even further and the newer
subregions (`bubbles`) 552 and 554 have become substantially larger. At
the same time, the subregion walls of original subregions 514 and 522 have
retracted to new configurations 514' and 522'. New subregions (or
`bubbles`) 562 and 564 have now appeared with magnetic orientations
pointing up instead of down. The expansion of new subregions (`bubbles`)
562 and 564 is discontinuous and nonsymmetrical just as the case with
older bubbles 552 and 554.
At the seventh snapshot S-557, the head to medium relative position has
advanced to Z.sub.R =7, and the underlying media excitation field 557 has
reached maximum intensity while continuing to point upwardly. The magnetic
orientations of domains in both the central (main) subregions and the left
and right edge subregions of GMR layers 510 and 520 now all point upwardly
in alignment with the underlying media excitation field 557. Domains in
the top edge subregions 513 and 523 of respective GMR layers 510 and 520
continue to point opposingly to the right and left respectively.
The magnetization conditions in seventh snapshot S-557 are now basically
the same as that in first snapshot S-551 except that magnetic orientations
in the central and edge subregions are upward rather than downward. The
response to a next flipping of the media excitation field will follow a
similar path as that of snapshots S-551 to S-557 except that the up/down
orientations will be reversed and rotations of the central subregions will
also be reversed.
Note that the angle difference between the central magnetic orientations,
511 and 521 is zero in seventh snapshot S-557 just as it was in snapshot
S-551 and the cosine of this angle difference is +1.0
(cos{0.degree.}=+1.0).
FIG. 5B plots the cosine-like response of the unbiased GMR element to
variation in the excitation field strength. This response, as seen, is a
function of the cosine of the angle difference between the magnetic
orientations of the central (main) subregions of the GMR layers 510 and
520 as those layers sweep through a transition of the media excitation
field. The resistance of the GMR element is roughly proportional to the
cosine of the angle difference between the magnetic orientations of the
central subregions of its alternating GMR layers.
The vertical axis of FIG. 5B shows magnetoresistive response to the medium
excitation field in terms of resistance or voltage. The horizontal axis of
FIG. 5B shows the strength of the medium excitation field in terms of
milli-EMU's (electromagnetic units) per centimeter squared. The field
strength position of 0 mEMU/cm.sup.2 corresponds to third snapshot S-553
in FIG. 5A. The field strength position of -1.2 mEMU/cm.sup.2 corresponds
to first snapshot S-551 in FIG. 5A. The field strength position of +1.2
mEMU/cm.sup.2 corresponds to seventh snapshot S-557 in FIG. 5A.
Note in FIG. 5B that the most nonlinear portion of the cosine-like response
curve (f[cos{180.degree.}]) is positioned over the field strength position
of 0 mEMU/cm.sup.2 (S-553). Note that this portion of the cosine-like
response curve (f[cos{180.degree.}]) is also the one with the minimum
slope. This means that the response of the unbiased GMR element to an
oscillating excitation field of very small magnitude is relatively
nonlinear and becomes more nonlinear as the magnitude of the oscillating
excitation field decreases. It also means that the gain of the unbiased
GMR element decreases as the magnitude of the oscillating excitation field
decreases. Both of these characteristics are disadvantageous, particularly
when one is trying to detect magnetic transitions of relatively low
strength.
FIG. 5C plots the output voltage or resistance of the non-biased GMR
element as the GMR element is excited by a sinusoidally oscillating
excitation field prerecorded on the medium. The vertical axis of FIG. 5C
shows magnetoresistive response to the external excitation field while the
horizontal axis shows position along a media track in terms of microns.
The pre-recorded excitation signal is assumed to be a square wave with a
period of 3 microns and a magnitude swinging symmetrically both positive
and negative (north and south).
The Z.sub.R position in FIG. 5C to the right of 0.0 microns corresponds to
snapshot S-551 of FIG. 5A. Response latency is due to hysteresis. The
Z.sub.R position to the right of 0.75 microns corresponds to snapshot
S-553 of FIG. 5A. The Z.sub.R position to the right of 1.5 microns
corresponds to snapshot S-557 of FIG. 5A. Note that the output at
approximately 0.0 microns (S-551) is undifferentiated from that at
approximately 1.5 microns (S-557) even though the excitation field is of
opposite polarity at these respective locations. It is not possible to
tell from the output of the GMR element whether the detected magnetic
transition is from down to up, or from up to down. Polarity information
within the prerecorded magnetic signal does not pass through to the output
of the unbiased GMR element.
FIGS. 2A through 2C respectively provide a set of `after`diagrams for the
case where the GMR element is biased by biasing element 180.
FIG. 2A shows magnetic orientations for the case of the medium excitation
field pointing fully down (snapshot S-251), for the case of the medium
excitation field being neutral (snapshot S-253) and for the case of the
medium excitation field pointing fully up (snapshot S-257). Down pointing
arrow 281 represents the biasing field. The biasing field is such that in
the neutral external-excitation state (snapshot S-253), the main (central)
magnetic orientations define a `scissor` configuration when overlaid on
top of one another. The major domain region of a first layer of the MR
element is preferably biased so as to be rotated +45.degree. relative to
its unbiased orientation by the bias field. The major domain region in an
adjacent second layer of the MR element is preferably biased so as to be
rotated -45.degree.relative to its unbiased orientation by the bias field.
This produces an angle difference of approximately 90 between the major
domain regions of the first and second layers due to the bias. The cosine
of this bias-induced angle difference (90.degree.) is approximately zero.
Note that in each of the magnetization states, S-251, S-253, S-257, of FIG.
2A (biased GMR element), the orientation of the edge subregions remain
substantially unchanged. There is essentially no flipping of domains at
the edges of the biased GMR element and thus the problem of random noise
being introduced due to the Barkhausen effect is obviated.
It is to be understood that the results shown in FIGS. 5B, 5C, 2B, 2C are
not pure cosine curves but rather the results of a computer simulation. In
the simulation set up, a tri-layer GMR element was assumed consisting of
magnetic/conductive/magnetic layers each 2 nanometers in thickness with a
Y-direction width of 0.5 micron and an X-direction length of 0.25 micron.
The medium signal was represented as having a perfect square pulse with
90.degree.rising and falling edges recorded thereon over a duration of 1.5
microns. A 30 nm magnetic spacing between the surface of the medium and
the head ABS was assumed. A permanent magnetic bias of strength MrT=0.45
memu/cm.sup.2 was assumed extending in the -X direction for the results
respectively labeled as "biased".
As seen in FIG. 5C (not-biased), the read-back voltage at the rising edge
of the square pulse (Z.sub.R =0.0 micron) is indistinguishable from the
read-back voltage at the falling edge of the square pulse (Z.sub.R =1.5
micron) because they are of the same polarity and magnitude. If an upward
spike of noise were introduced at the time of say, Z.sub.R =1.0, it would
be difficult to separate signal from noise.
As seen by contrast in FIG. 2C (biased), the read-back voltage at the
rising edge of the square pulse (Z.sub.R =0.0 micron) is clearly
distinguishable from the read-back voltage at the falling edge of the
square pulse (Z.sub.R =1.5 micron) because they are of opposite polarity.
If an upward spike of noise were introduced at the time of say, Z.sub.R
=1.0, it would be relatively easy to separate signal from noise using
alternating polarity as a basis for filtering.
Note in FIG. 2B that the most linear portion of the cosine-like response
curve (f[cos{90.degree.}]) is positioned over the field strength position
of 0 mEMU/cm.sup.2 (S-253). Note that this portion of the cosine-like
response curve (f[cos{90}]) is also the one with the maximum slope. This
means that the response of the biased GMR element to an oscillating
excitation field of very small magnitude is relatively linear and becomes
more linear as the magnitude of the oscillating excitation field
decreases. It also means that the gain of the biased GMR element increases
as the magnitude of the oscillating excitation field decreases. Both of
these characteristics are highly advantageous, particularly when one is
trying to detect magnetic transitions of relatively low strength.
The comparative transfer curve characteristics for the non-biased and
biased cases is best seen in FIG. 6 where the circles represent the
non-biased arrangement and the triangles represent the biased arrangement.
The region of key interest is that about the field strength equals zero
axis (dashed vertical line). An oscillatory excitation field of relatively
low strength will
provide a much more linear response and a greater output swing for the case
of the biased GMR element as compared to that of the non-biased GMR
element, all other factors being equal.
The biasing field (281 of FIG. 2A) can be provided by a permanent magnet,
or a combination of a pre-oriented exchange layer and a soft layer, or an
electric current source which induces the magnetic bias field, or a
combination of one or more of these bias-providing means. The permanent
magnet can be used in the case where there is no strong write fields to
permanently disturb the magnetization of the magnet. For the case where
strong write fields are expected to pass through the bias providing means,
use of a recoverable biasing system such as a combination of a
pre-oriented exchange layer (antiferromagnetic layer) and a soft layer
(ferromagnetic layer) is preferred.
Referring to FIG. 3A, a method for manufacturing a biased GMR head in
accordance with the invention will be described. Where possible, like
reference numerals in the "300" number series are used for components
having like counterparts numbered in the "100" series in FIGS. 1A and 1B.
The following layers are deposited by sputtering, or CVD (chemical vapor
deposition) or other appropriate deposition methods onto a ceramic
substrate 310 preferably in the recited order: an optional base insulating
layer 311 made of an EN/MN material; a first pole/shield layer 321 made of
an EC/MC material; a first contact layer 322 made of an EC/MN material; a
GMR layer 323 made of alternating ultra-thin films of EC/MN and EC/MC
materials; a second contact layer 324 made of an EC/MN material; and a
patterned first photoresist layer 302 having a photolithographically
defined aperture 301 extending vertically therethrough.
Referring next to FIG. 3B, aperture 301 is used to form a cavity 303
extending through layers 322 through 324. (For illustrative brevity, the
optional base insulating layer 311 is no longer shown but is understood to
be formed, if present, on the top surface of substrate 310.) The Y
direction width of the cavity 303 is equal to or greater than that of the
to-be-formed GMR element at the left side of the cavity. The X direction
length of the cavity 303 is preferably longer than the ultimate length of
the GMR element and more preferably 5 to 10 times the ultimate length. The
cavity 303 can be milled using for example, argon ion milling or reactive
ion etching (RIE), the methods being picked as appropriate for the
material of each of layers 322-324. The cavity formation will typically
have some isotropic side cutting, and as such sloped sidewalls may result
as illustrated. The slope tangent is roughly in the range 1:1 to 2:1
(delta Z: delta X).
In FIG. 3C, a thin film of EN/MN material such as RF-sputtered Al.sub.2
O.sub.3 (aluminum oxide) is deposited. Part of the EN/MN material adheres
to the top of layer 321 and the sidewalls of layers 322-324 as illustrated
at 305. The remainder adheres to the top of the first photoresist layer
302 as shown at 304. The inside-cavity portion 305 of the EN/MN thin film
will be referred to below as the first insulating layer 305.
When the next-described antiferromagnetic material 306-307 is made to be
electrically insulating, then the first insulating layer 305 may be
omitted.
In FIG. 3D, a thin film of antiferromagnetic material such as NiCoO or MnFe
or TbCo or MnNi is deposited as indicated at 306 and 307. The
cavity-internal portion 307 of the antiferromagnetic thin film will be
referred to below as the exchange layer 307. The magnetic domain
orientations of the exchange layer 307 are directed along the X direction
by applying a like-oriented external field either during deposition or
during a post-deposition high-temperature anneal.
In FIG. 3E, a thin film of soft ferromagnetic material such as Permalloy is
deposited as indicated at 308 and 309. The cavity-internal portion 309 of
the ferromagnetic thin film will be referred to below as the bias layer
309. Note that the bias layer 309 is at least partially in the XY plane of
the GMR layer 323. Antiferromagnetic coupling across the XY boundary of
the exchange layer 307 and the bias layer 309 causes the magnetic domains
of the bias layer 309 to orient themselves along the X direction, thus
applying a magnetic bias in the X direction to the GMR element 323. The
magnetic bias might be temporarily disrupted when write fields are
generated between the first pole/shield layer 321 and the to-be-formed
third pole/shield layer 326 (FIG. 3K) but the orientation of the exchange
layer 307 is unaffected and snaps the orientation of domains in the bias
layer 309 back into the X direction after writing (recording) completes.
In cases where the device is not used for magnetic recording (writing) or
the magnetic write fields are of sufficiently low intensity not to
neutralize it, a permanent magnet (not shown) may be used in place of the
combination of the exchange layer 307 and the bias layer 309.
As long as the bias layer 309 is at least partially in the XY plane of the
GMR layer 323, it makes little difference in terms of operation whether
bias layer 309 is on top of the exchange layer 307 or vise versa. For some
cases it may be advantageous to reverse the deposition order, putting the
bias layer 309 down first and the exchange layer 307 down second, on top
of the bias layer 309. The soft magnetic material of the firstly laid-down
bias layer 309 can provide improved crystallographic boundary alignment
for the secondly deposited exchange layer 307 in such a case.
For the case where the exchange layer 307 is made of an electrically
conductive material, and the bias layer 309 is made of an insulating
material, it may be advantageous to eliminate the deposition of one or
both of the first insulating layer 305 (FIG. 3C) and a below-mentioned
second insulating layer 313 (FIG. 3F), however both insulating layers, 305
and 313, should be employed in order to best assure that electrical
current will not leak through the biasing element 180 (FIG. 1A).
In FIG. 3F, a further magnetically and electrically nonconductive material
(EN/MN) is deposited, the portion passing through first photoresist layer
301 forming a second insulating layer 313 and having a top surface
approximately coplanar with the top surface of the second contact layer
324 (FIG. 3A). The remainder of the EN/MN material forms on top of the
bias layer material 308 as shown at 312.
Referring to FIG. 3G, an appropriate solvent is next used to dissolve the
first photoresist layer 302 and float-away the overlying materials
indicated in FIG. 3F as 304, 306, 308 and 312. This leaves behind the
generally planar structure having the second contact layer 324 and second
insulating layer 313 forming its upper surface.
Referring to FIG. 3H, a second pole/shield layer 325 is deposited. A second
photoresist layer 334 is thereafter deposited and patterned as shown to
leave exposed the back portion of the structure while masking the front
portion.
Referring to FIG. 3I, the second photoresist layer 334 is used for milling
away the material previously occupying area 335. The milling operation
cuts approximately to the top of the first pole/shield layer 321 or
slightly below. One or a combination of argon ion milling and RIE and
other anisotropic etching methods may used, as appropriate in accordance
with the materials chosen for layers 321-325 and 305-313, for forming the
cutout 335.
The second photoresist layer 334 (FIG. 3H) is next removed to leave the
structure shown in FIG. 3I.
Referring to FIG. 3J, a third insulating layer 336 made of an EN/MN
material is next deposited on the structure of FIG. 3I and planarized. A
third photoresist layer 337 is thereafter deposited onto the third
insulating layer 336 top of second pole/shield layer 325 and patterned to
define the Y-direction width of the stem portion of the I-beam front view
of FIG. 1B.
Referring to FIG. 3K, the EN/MN side support material 170 of FIG. 1B is
next deposited and the structure is again planarized. Thereafter, a
further photoresist layer (not shown) is deposited and patterned to
straddle in the Y-direction the structure shown in FIG. 3K so that the
material of the third pole/shield layer 326 may be deposited as shown. The
third pole/shield layer 326 is composed of the same or a substantially
similar composition as that used for the first and second pole/shield
layers, 321 and 325.
Other portions of the head, such as the planar write coil (140) may be
formed after or during the above steps in accordance with known methods.
It is to be understood that in a mass production environment, the
structure of FIGS. 3A-3K is reproduced many times across each of plural
wafers and individual read/write heads are diced out from each wafer at
the end of the process. Accordingly, an economic method for mass-producing
compact, CPP-type, biased magnetoresistive heads is provided.
FIG. 4 provides a closer view of the mechanism by which a GMR element 423
is biased even though it is periodically exposed to write flux transitions
that tend to demagnetize it. Exchange layer 407 is preoriented so that the
magnetic domains of its antiferromagnetic material point opposingly in the
X direction (in the direction of a line extending from the GMR element 423
to an adjacent medium surface (not shown). Surface coupling at the
boundary 497 between the exchange layer 407 and the ferromagnetic bias
layer 409 produces magnetic domains pointing unidirectionally along the X
direction within the ferromagnetic bias layer 409. The flux of these
unidirectionally pointed domains extends across boundary 493 into the GMR
element 423 to thereby bias the GMR element 423. The orientation of
opposingly directed domains within the antiferromagnetic exchange layer
407 are not permanently disturbed by application of write flux, and hence
the bias is maintained during read mode even if the head had been
previously used to generate relatively strong write pulses.
It should be understood that the invention is not limited to the specific
parameters and materials described above. Various modifications and
variations may be made within the scope of the invention.
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